Effect of Low-Molecular-Weight Organic Acids on the Release of Phosphorus from Amended Calcareous Soils: Experimental and Modeling

Calcareous soils usually exhibit low phosphorus availability. Low-molecular-weight organic acids are known to increase the release of phosphorus, but the release of phosphorus when low-molecular-weight organic acids, along with organic and inorganic amendments that coexist, is still poorly understood. The phosphorus release was studied with two soils: a clay loam and a sandy loam soil. These two soils were amended with three different organic and inorganic products at a rate of 5%, i.e., sugar beet bagasse ash, poultry manure, and sewage sludge. The phosphorus release was examined using a wide range (0, 0.1, 1, 2.5, 5, 10, 30, 50, 70, and 100 mM) of ubiquitous low-molecular-weight organic acids (citric, oxalic, and malic acids). The results showed that the ability of low-molecular-weight organic acids to release phosphorus followed the order citric > oxalic > malic. The release of phosphorus due to the application of low-molecular-weight organic acids from amended soils followed the order poultry manure > sewage sludge > sugar beet bagasse ash. The release of phosphorus from surface of calcium carbonate and, to a lesser extent, the surfaces of iron/aluminum oxides may be the primary cause of the release of phosphorus at low concentrations of low-molecular-weight organic acids, whereas calcium phosphate mineral dissolution is the primary cause of phosphorus release at high concentrations of low-molecular-weight organic acids. The PHREEQC program relatively well simulated the phosphorus release and equilibrium pH in different treatments. The release of phosphorus from low-molecular-weight organic acid–driven soil was influenced by the kind of low-molecular-weight organic acids, their concentrations, soil types, and the type of amendments.